This invention relates to high frequency transformer apparatus for coupling single ended high frequency transmission lines (e.g. unbalanced lines) to a pair of balanced transmission lines, commonly referred to as a balun, and, more specifically, to a planar form of balun for application in a monolithic microwave integrated circuit (xe2x80x9cMMICxe2x80x9d).
In high frequency RF circuits it is common to convert or split a high frequency RF signal supplied over a two-wire transmission line into separate balanced signals, equal in power and out of phase by one hundred and eighty degrees, and allow the separate signals to propagate along separate transmission paths. Formed of two wires, one of which is connected to electrical ground, the two-wire transmission line (and, hence, the RF signal) is seen as unbalanced with respect to ground, while the latter two transmission paths (and the two derived signals) are balanced with respect to that ground. Such a conversion of unbalanced to balanced signals is often accomplished by a Balun transformer. Conversely, some implementations of Balun transformers also permit the reverse action, converting a balanced signal into an unbalanced pair. In general, a Balun transformer (generally referred to simply as a Balun) is either active or passive in character. The passive type does not require an external source of electrical power for operation; only the high-speed signals, RF, of interest are required for the conversion. Passive Baluns often possess bidirectional characteristics. That is the signals of interest may be either inputs or outputs to any of the ports of the Balun. The present invention relates to Baluns of the passive type and, more particularly, to Baluns used in the unbalanced to balanced direction that find typical application in mixer frequency downconverters for both the local oscillator (xe2x80x9cLOxe2x80x9d) and RF signals.
Many forms of Baluns are known in the art. Examples of Balun structures are found in patents U.S. Pat. No. 5,428,838 to Chang et al, U.S. Pat. No. 5,819,169 To Faden, U.S. Pat. No. 5,061,910 to Bouny, and U.S. Pat. No. 5,428,840 to Sadir. Often the Balun is integrated within the structure of another active high frequency device, such as a ring mixer or star mixer. The mixer device in turn forms a component of a Microwave Monolithic Integrated circuit (xe2x80x9cMMICxe2x80x9d) device. MMIC devices by definition contain all the active and passive circuit elements and associated interconnections formed either in site on or within a semi-insulating semiconductor substrate or insulating substrate by one or more well known deposition processes.
Traditional coupled-line balun transformers implemented monolithically have typically been realized in a multi-substrate layered microstrip or stripline process or have been constructed in a manner unique to a particular application. Examples of the latter are the Star mixer described in the cited ""838 Chang et al patent; and the high leakage and the intermodulation suppression ring mixer described in the cited ""169 Faden patent. Multi-substrate layer processes are expensive, and may not be available or standard at every semiconductor foundry. As an advantage, the present invention does not require multi-substrate layer processes.
The ""838 Chang et. al. patent illustrates a diode star mixer which incorporates an identical pair of coupled line baluns oriented at right angles to one another and which is capable of configuration in a MMIC circuit. Each balun is formed of coupled transmission line microstrips (FIG. 3). A straight center microstrip formed on a substrate of semiconductor material, such as Gallium Arsenide (dielectric constant 12.9), or on a substrate of insulating material, such as Alumina (dielectric constant 9.9), is bounded on both sides of the length thereof by two pairs of identical microstrips with one end of that center microstrip serving as an input and the other end being xe2x80x9copenxe2x80x9d, that is, unconnected. One pair of the microstrips bounds essentially one-half of the length of the center microstrip and the other pair bounds essentially the remaining half of the length of the center microstrip. The outer ends of the two microstrips of each pair are connected to ground, while the inner ends of the two microstrips of each pair are electrically connected together to form first and second outputs. One end of the center microstrip serves as an input for the unbalanced line, while the remaining end of that microstrip remains open, that is, is not directly electrically connected to anything else.
In the practical embodiment of the star mixer illustrated and described in detail in the Chang patent, the balun is shown as an integral element of a dual balun structure in which the baluns are oriented perpendicular to one another and the center connectors of the two baluns are connected together where they criss-cross. The balun of the ""838 Chang et al patent appears to offer a balun structure that is useful at those very high frequencies at which the length of the straight microstrip transmission lines remains practical. However, as one realizes, should the star mixer be designed for lower frequencies, such as approximately 2 GHz, the length of the transmission lines require a greater space, which, following the structure defined in the ""838 Chang patent, is impractical for and could not be effectively implemented within a MMIC structure. As an advantage, the present invention is more compact in size than the baluns of the Chang patent and is practical in MMIC structures at those low frequencies. A Star or Ring mixer implemented with the present invention occupies significantly less real estate on the substrate than that of the ""838 Chang patent at any range of frequency, and provides comparable performance. Because of the requirement for less space, as a further advantage, the present invention permits greater miniaturization of MMIC circuits than that of the Chang patent even at those higher frequencies at which the mixer of the Chang patent remains practical.
According to the Chang patent, coupled line baluns, the type found in the Chang patent and in the present invention, will generally perform poorly unless the coupled lines have high even-mode impedance and the even and odd mode phase velocities are closely matched. Inherent to the unique construction of the Balun of the Chang patent and to that of the present invention is that both Baluns are tolerant of low even-mode impedances. Due to that tolerance it is possible to use the baluns in the construction of Star and Ring mixers. When constructed on a high dielectric base or substrate, as is typically the case in MMIC applications, adequate even and odd mode phase velocity matching is also achieved.
Accordingly, an object of the invention is to provide a Balun construction that provides balanced anti-phase outputs over an ultra-wide frequency range;
A further object of the invention is to provide a Balun structure that for a given set of comparable performance parameters occupies less space than the prior art Baluns;
A still further object of the invention is to provide a planar physical construction for a balun that is of application within MMIC devices and may be scaled for use over various ranges of frequencies, as example, 3 to 6 GHz, 12 to 24 GHz and 20 to 40 GHz frequency ranges.
And a still further object of the invention is to provide a new Balun structure that is essentially planar in shape and may be fabricated on a single layer substrate, either as part of a MMIC device or separately.
In accordance with the foregoing objects and advantages, the invention is characterized by two pairs of coupled microstrip spiral coils attached to the flat upper surface of an electrical insulating substrate with one pair of coils located side by side with the other pair. Each coil in a pair is interleaved with the other coil in the pair and is spaced from one another and the coils of the pair are electro-magnetically linked or coupled. The coils of one pair define a spiral of decreasing radius, the coils of the other pair define a spiral of increasing radius, and the one pair of coils is a mirror image of the other pair of coils. One coil in each pair is serially connected by an air bridge with one coil of the other pair to serve as series connected primary windings of the balun; and one end of the second coil in the foregoing series connected primary windings is open. An end of each of the remaining coils in each pair are connected to a common juncture, and is directly or indirectly grounded, while the remaining ends of the latter two coils define the balanced outputs of the balun transformer. Geometrically, the coils are typically realized in a circular or rectangular spiral configuration.
The present invention provides a coupled line balun that is multi-purpose, ultra-wideband, compact in size, planar, monolithic, and inexpensive. The invention is suitable for many applications, including as a component of microwave mixers, frequency multipliers and balanced amplifiers.
The foregoing and additional objects and advantages of the invention together with the structure characteristic thereof, which was only briefly summarized in the foregoing passages, will become more apparent to those skilled in the art upon reading the detailed description of a preferred embodiment of the invention, which follows in this specification, taken together with the illustrations thereof presented in the accompanying drawings.